In ink-jet printing methods, droplets of ink are directly projected onto a printing medium from very fine nozzles and allowed to adhere to the printing medium to obtain printed materials on which characters or images are printed. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the printing medium, non-contact with printed characters or images, etc. In recent years, in order to impart good weathering resistance and good water resistance to the printed materials, an ink containing a pigment as a colorant has been extensively used.
On the other hand, it has been required to print characters or images on a printing medium for commercial printing purposes using a low-liquid absorbing coated paper such as an offset-coated paper, or a non-liquid absorbing resin film such as a polyvinyl chloride resin film, a polypropylene resin film and a polyester resin film.
It is known that when characters or images are printed on the low-liquid absorbing or non-liquid absorbing printing media by the ink-jet printing methods, there tend to occur problems such as prolonged drying time owing to slow or no absorption of liquid components, and deterioration in rub fastness early after printing. Also, it is known that unlike a plain paper in which a pigment is easily penetrated, the low-liquid absorbing or non-liquid absorbing printing media tend to suffer from deposition of pigment particles remaining thereon which are directly susceptible to an external force, so that the characters or images printed on these printing media tend to be deteriorated in rub fastness even after being dried.
In order to improve rub fastness of the printed characters or images after being dried, there are known technologies for incorporating rein particles having a film-forming capability into the water-based ink.
For example, JP 2015-48466A (Patent Literature 1) discloses a process for producing a water-based ink for ink-jet printing which is excellent in storage stability as well as rub fastness and solvent resistance when printed on a low-water absorbing printing medium, said process including the steps of subjecting a monomer mixture containing (meth)acrylic acid and a (meth)acrylic acid ester in a total amount of not less than 90% by mass to emulsion polymerization to obtain an emulsion A containing acrylic acid-based copolymer particles having a weight-average molecular weight of not less than 100,000; adding and dispersing a pigment into the emulsion A to obtain a pigment dispersion; subjecting a monomer mixture containing (meth)acrylic acid and a (meth)acrylic acid ester in a total amount of not less than 90% by mass to emulsion polymerization to obtain an emulsion B containing acrylic acid-based copolymer particles having a weight-average molecular weight of not less than 100,000; and mixing the pigment dispersion, the emulsion B and an organic solvent with each other to obtain the water-based ink.
In addition, JP 2012-201691A (Patent Literature 2) discloses an ink composition that is excellent in storage stability, ejection properties, printability for production of printed materials, rub fastness, water resistance, solvent resistance, etc., and has good sustainability against not only rubbing between the printed materials upon continuous printing but also contact with water or an alcohol-based solvent, and further exhibits good ejection properties, in which the ink composition contains resin particles having an average particle size and a coarse particle content which fall within respective specific ranges, and a humectant solvent.